In recent years there has been active research in the area of nanosecond-type pulse generation. Such research has produced devices that utilize high power photoconductive solid state switches coupled to energy storage devices. In order for such a device to produce a nanosecond-type pulse, its photoconductive switch must have the ability to transition from a high resistivity state to a conductive state in a sub-nanosecond time interval. One such switch, disclosed in U.S. Pat. No. 5,028,971, issued to Anderson H. Kim et al on Jul. 2, 1991, entitled, "High Power Photoconductor Bulk GaAs Switch" is incorporated herein by reference.
This GaAs switch is comprised of two, mutually opposite, gridded electrodes separated by a GaAs substrate capable of electrical energy storage. The stored energy can be photoconductively discharged when it receives laser light. More specifically, when the laser light is applied to the semiconductor material, electron hole pairs are generated in the substrate, thus causing the electrical resistance of the semiconductor material to instantaneously decrease. This instantaneous resistance change causes the stored energy to convert into discharge current and flow through an output circuit such that an RF pulse is radiated in a direction perpendicular to the substrate.
It is widely recognized that the shorter the RF radiator's pulsewidth becomes, the wider its radiation bandwidth will be. Hence, the faster the radiated pulse's rise time becomes, the wider the radiation bandwidth will be. Consequently, it has become very desirable for those skilled in the art to construct devices capable of generating pulses having faster and faster rise-times so that the radiation bandwidth can be extended further and further.
The critical element in generating such fast rise time, high voltage pulses is the energy storage device itself. Heretofore, there are two general energy storage techniques used to generate faster rise-time, high power pulses.
The first technique is to create a device that utilize the recombination property of semiconductor material. It has been determined, however, that such semiconductor materials exhibit a slow switch recovery time at high voltages. The long recovery time has been attributed to both the switch lock-on phenomena and the substantially long recombination time attributable to gallium arsenide. Hence, devices utilizing this storage technique are not desirable for the many wideband applications that require such high power pulses.
The second technique utilizes an energy storage element comprised of either a short section of transmission line or a capacitor that can be photoconductively triggered to instantaneously discharge all, or substantially all, of its stored energy to a load. As with the aforementioned technique, the extended recovery time inherent in a device utilizing such a photoconductive switch prevents this device from producing extended wideband radiation.
A major breakthrough in the generation of narrow pulses, however, was disclosed in the inventors U.S. Pat. No. 5,227,621 entitled "Ultra-Wideband High Power Photon Triggered Frequency Independent Radiator," issued to Kim et al. Jul. 13, 1993 and incorporated herein by reference. As disclosed, this frequency-independent radiator combines energy storage and antenna radiating functions into one structure to create an ultra-wideband frequency radiator capable of generating pulses with a range of frequency components from hundreds of megahertz to several gigahertz. Basically, this radiator utilizes two identical quasi-radial transmission line structures to store electric energy while it simultaneously implements photoconductive switching to trigger the instantaneous discharge of the stored energy to generate the desired ultra-wideband RF radiation.
Such an energy storage device comprises a dielectric storage medium, two quasi-radially shaped, metalized electrodes mounted opposite one another on the top surface of the dielectric storage medium, and a metalized electrode mounted on the bottom surface of the dielectric medium. The two quasi-radial shaped electrodes are connected to the bottom electrode via a photoconductive switch centrally located on the dielectric. When the switch is activated by laser radiation, the stored energy discharges through a predetermined load such that a sub-nanosecond type pulse is generated.
Those skilled in the art have recognized that the shape and overall geometry of the device directly affects the width of the discharged pulse, and thus its bandwidth. Specifically, the shape of the electrodes, the position of the energy storage elements, and the position of the photoconductive switches, directly affect the charging and discharging characteristics of the stored energy.
It has also been recognized that the gap distance between the electrodes directly affects the bandwidth of the radiated pulse. The narrower the gap the greater the radiated bandwidth. If the gap is made too small, however, device flashover, and thus device breakdown, may occur. Consequently, device efficiency is directly limited by the geometry of the storage element.
A radiator incorporating a storage element with an innovative geometry to achieve an even greater bandwidth than the prior art was disclosed in the inventor's co-pending application entitled "Ultra-wideband High Power Photon Triggered Frequency Independent Radiator With Equiangular Spiral Antenna," Ser. No 08/064,525, and incorporated herein by reference. This device utilized an equiangular spiral antenna electrode (in place of the quasi-radial transmission line disclosed above) positioned on the surface of a photoconductive semiconductor substrate. The spiral antenna electrode was positioned such that it could store high power electrical energy to be instantaneously discharged upon photon triggering. Consequently, the energy storage and energy radiation functions are performed in the same section of the device (i.e. spiral antenna). The result is a device that radiates RF energy at a much wider bandwidth than previously disclosed without compromising the radiated field strength.
Although RF generators utilizing such a device geometry can radiate energy having increased bandwidth and improved performance over existing devices, those skilled in the art still desire and recognize the need for Rf generators utilizing new and innovative geometric shapes and schemes that provide for even greater device performance and efficiency while not adding to the device's overall size or cost.